Field of the Invention
The present invention concerns a method to determine a magnetic resonance control sequence which has at least one first pulse arrangement that acts in a spatially selective manner in a first selection direction, and a subsequent second pulse arrangement that acts in a spatially selective manner in a second selection direction. Moreover, the invention concerns a method to operate a magnetic resonance system with such a magnetic resonance control sequence, a control sequence determination device in order to determine such a magnetic resonance control sequence, and a magnetic resonance system with such a control sequence determination device.
Description of the Prior Art
In a magnetic resonance system (also called a magnetic resonance tomography system), the body to be examined is typically exposed to a relatively high basic magnetic field (B0 field), for example of 1, 5, 3 or 7 Tesla, with the use of a basic field magnet. A magnetic field gradient is additionally applied by a gradient system. Via a radio-frequency transmission system, radio-frequency excitation signals (RF signals) are then emitted (radiated) by suitable antennas, which cause nuclear spins of specific atoms to be excited to resonance by the associated radio-frequency field (B1 field). This excitation can be described as the nuclear spins being flipped (deflected) by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals, known as magnetic resonance signals, are radiated that are then received (detected) by suitable reception antennas and are then processed further. The desired image data can be reconstructed from the raw data acquired in such a manner.
For a defined measurement (data acquisition), a pulse sequence with a radio-frequency pulse train is emitted and a gradient pulse train (with matching gradient pulses in the slice selection direction, in the phase encoding direction and in the readout direction) is switched (activated) in coordination with the radio-frequency pulse train. For imaging, the timing within the sequence, i.e. the time intervals that pulses follow one another, is particularly significant. A number of control parameter values is normally defined in a sequence known as a measurement protocol, which is created in advance and retrieved (for example from a memory) for a defined measurement, and which can be modified as necessary on site by the operator, who can provide additional control parameter values (for example a defined slice interval of a stack of slices to be measured, a slice thickness, etc.). A magnetic resonance control sequence is then calculated on the basis of all of these control parameter values. This magnetic resonance control sequence is also designed as a measurement sequence, “MR sequence” (magnetic resonance sequence), or shortened to just “sequence”.
In the classical procedures, the acquisition of images of the interior of the object takes place slice by slice. Nuclear spins in a relatively thin slice are excited, which is referred to as the slice being individually excited, typically a slice between 1 and 5 mm in thickness. Such a selective excitation is achieved by activating a gradient in the slice selection direction in coordination with the radiation of a radio-frequency excitation pulse. Such a pulse arrangement (including the exciting radio-frequency pulse and the associated gradient) causes the radio-frequency pulse to act only selectively on the region defined by the gradient. In most cases, this slice selection direction proceeds parallel to the axis that is commonly defined as the z-axis (the longitudinal axis of the scanner, which is also the longitudinal axis of a patient lying in the scanner. Spatial coding within a slice then takes place by phase encoding in one direction (most often the y-direction) and by a readout coding in another direction (most often the x-direction). In this way, a two-dimensional frequency domain (known as k-space) is filled by entering the raw data at designated points in a memory. An image of the slice is created from the k-space data by a two-dimensional Fourier transformation.
It is also possible to excite nuclear spins in, and acquire MR signals from, larger three-dimensional volumes, in a 3D method. In such methods rather than a thin slice, a relatively thick slice (typically designated as a “slab”) is excited in an excitation process. The raw data from these slabs (most of which are more than 10 mm thick) are acquired again with spatial resolution in the slice selection direction. This typically takes place via a second phase encoding, meaning that data acquisition takes place in such methods with phase encoding in two directions and readout encoding in one direction, in order to thus fill three-dimensional k-space with raw data, and to generate a three-dimensional image volume therefrom via a 3D Fourier transformation.
Since the phase encoding steps during the data acquisition essentially define the total acquisition time for the raw data, it is advantageous to operate with optimally few phase encoding steps. It is nevertheless necessary, k-space must be covered (filled) sufficiently densely enough (meaning that a sufficient sampling must take place), since otherwise aliasing could occur. In order to sample sufficiently densely, the length of the volume to be excited should be optimally short in each of the phase encoding directions, such that the volume in the spatial domain completely encompasses the region of interest. In the slice selection direction, the volume is established by the boundaries of the slab, which means that the slab thickness and thus the length of the volume that is to be covered by the phase encoding in a slice selection direction can be determined by the selection of the first pulse arrangement. However, normally the entire subject width must be considered in the second phase encoding direction orthogonal to the slice selection direction. In order to also limit the width in this direction, it has been proposed to emit, with the radio-frequency excitation pulse, an additional pulse arrangement that acts selectively in a second direction (namely a refocusing pulse with corresponding gradients switched in the second direction) after the first pulse arrangement that selectively acts in the slice selection direction. For example, this method, also designated as “inner volume refocusing”, is described by D. A. Feinberg, J. C. Hoenninger, L. E. Crooks, L. Kaufman, J. C. Watts, and M. Arakawa in “Inner volume MR imaging: technical concepts and their application,” Radiology 156, 743-747, 1985.
A viewing volume in two directions is thereby selectively bounded, and improvements with regard to the artifacts such as aliasing can already be achieved in both directions in spite of the reduction of the phase encoding steps. However, it has turned out that aliasing artifacts or even signal cancelations can still occur, such as from fat tissue due to the chemical shift, particularly if a larger field of view is selected.